The interaction between soft optic and acoustic phonons was investigated for Sn2P2(Se0.28S0.72)6 proper uniaxial ferroelectrics by Brillouin scattering and ultrasonic pulse-echo techniques. The elastic softening of hypersound velocity of transverse acoustic phonons and for both longitudinal and transverse ultrasound waves which propagate near direction of the modulation wave vector (in the incommensurate phase at x > xLP ) was found at cooling to the Lifshitz point in the paraelectric phase. The strong increase of the ultrasound attenuation have also been observed. Such phenomena are related to the linear interaction of the soft optic and acoustic branches in the region of relatively short-range hypersound waves and to the strongly developed long-range order parameter fluctuations in the ultrasound frequency range. The hypersound velocity temperature dependence was described within the Landau-Khalatnikov approximation for the ferroelectric phase.

The temperature dependence of acoustic properties of the Sn(2)P(2)(Se(x)S(1-x))(6) uniaxial ferroelectric in the vicinity of the Lifshitz point (LP) was investigated by Brillouin spectroscopy and analysed in the Landau-Khalatnikov approximation. An anomalous decrease of the longitudinal hypersound velocity in the paraelectric phase caused by fluctuation effects and crystal structure defects has been found near the LP. Besides this, a small softening of the transverse acoustic phonons is observed, which is due to their linear interaction with the soft optic mode found for incommensurate phase transitions in proper ferroelectrics. The lattice instability analysis of Sn(2)P(2)S(6) and Sn(2)P(2)Se(6) crystals and their solid solutions in a polarizable ion model shows that a possible reason for the absence of a soft acoustic mode is nonorthogonality of the spontaneous polarization vector and modulation wavevector which both lie in the monoclinic symmetry plane.

Abstract:The temperature and frequency dependencies of sound attenuation for the proper uniaxial ferroelectric Sn 2 P 2 S 6 , which has a strong nonlinear interaction of the polar soft optic and fully symmetrical optic modes that is related to the triple well potential, were studied by Brillouin spectroscopy. It was found that the sound velocity anomaly is described in the Landau-Khalatnikov approximation with one relaxation time. For explanation of the observed temperature and frequency dependencies of the sound attenuation in the ferroelectrric phase, the accounting of several relaxation times is needed and, for quantitative calculations, the mode Gruneisen coefficients are more appropriate as interacting parameters than are the electrostrictive coefficients. Relaxational sound attenuation by domain walls also appears in the ferroelectric phase of

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In Sn2P2S6 ferroelectric under compression, the second‐order phase transition line is observed down to the tricritical point as the transition temperature decreases to 250 K. A tricriticality at a similar temperature is also revealed in the mixed crystals on S→Se substitution. For the mixed crystals with Sn→Pb substitution, ultrasound, hypersound, and low‐frequency dielectric studies also show heterophase features appearing on decrease of the ferroelectric transition temperature below a so‐called “waterline temperature” near 250 K. Such behavior agrees with the Blume–Emery–Griffiths (BEG) model appropriate for the ferroelectric system under investigation with a three‐well local potential for the order parameter fluctuations. The BEG model modified by the random‐field phase diagram in the vicinity of the tricritical point qualitatively explains the influence of the cationic substitution on the (PbySn1−y)2P2S6 ferroelectric properties.